A heat recovery steam generator (HRSG) is an energy recovery heat exchanger that recovers heat from a hot gas stream. It produces steam that can be used in a process (cogeneration) or used to drive a steam turbine (combined cycle). Heat recovery steam generators generally comprise four major components—the economizer, the evaporator, the superheater and the water preheater. In particular, natural circulation HRSG's contain an evaporator heating surface, a drum, as well as piping to facilitate an appropriate circulation rate in the evaporator tubes. A once-through HRSG replaces the natural circulation components with the once-through evaporator and in doing so offers in-roads to higher plant efficiency and furthermore assists in prolonging the HRSG lifetime in the absence of a thick walled drum.
An example of a once-through evaporator heat recovery steam generator (HRSG) 100 is shown in the FIG. 1. In the FIG. 1, the HRSG comprises vertical heating surfaces in the form of a series of vertical parallel flow paths/tubes (disposed between the duct walls 111) configured to absorb the required heat to form a first heat exchanger 104 and a second heat exchanger 108. In the HRSG 100, a working fluid (e.g., water) is transported to an inlet manifold 105 from a source 106. The working fluid is fed from the inlet manifold 105 to an inlet header 112 and then to the first heat exchanger 104, where it is heated by hot gases from a furnace (not shown) flowing in the horizontal direction. The hot gases heat tube sections of the first and second heat exchangers 104 and 108 disposed between the duct walls 111. A portion of the heated working fluid is converted to a vapor and the mixture of the liquid and vaporous working fluid is transported to the outlet manifold 103 via the outlet header 113, from where it is transported to a mixer 102, where the vapor and liquid are mixed once again and distributed to the second heat exchanger 108. This separation of the vapor from the liquid working fluid is undesirable as it produces temperature gradients and efforts have to be undertaken to prevent it. To ensure that the vapor and the fluid from the heat exchanger 104 are well mixed, they are transported to a mixer 102, from which the two phase mixture (vapor and liquid) are transported to the second heat exchanger 108 where they are subjected to superheat conditions. The second heat exchanger 108 is used to overcome thermodynamic limitations. The vapor and liquid are then discharged to a collection vessel 109 from which they are then sent to a separator 110, prior to being used in power generation equipment (e.g., a turbine). The use of vertical heating surfaces thus has a number of design limitations.
A common design consideration for boiler equipment is of the number of cold, warm, and hot starts a plant can accommodate over a period of time. The specific combination of these conditions directly relates to the equipment lifetime due to the adverse effects inherent in the daily thermal cycling of thick-walled pressure vessel equipment subjected to these drastic temperature changes. Often, thick walled equipment begins to fail as a result of prolonged thermal cycling. To prevent such failure, critical equipment must be identified and evaluated to ensure that operational demand can be satisfied. These evaluations necessitate additional inspections and maintenance, resulting in the loss of time and productivity.
It is also desirable to have as much operational flexibility as is desirable for combined cycle power plants because these power plants are often shut down and restarted as electrical power demand varies. The addition of renewable energy sources such as solar and wind increases the need to shut down and restart combined cycle power plants due to the variation in power output from such renewable resources. Stresses in various components of the HRSG due to thermal transients during these startups can limit the total number of times the heat recovery steam generators can be shut down and started over its operational life. It is therefore desirable to reduce the temperature transients in the components associated with the HRSG.